Deposits related to chemical sedimentation Sedimentary Iron Ores. GLY 361 Lecture 14

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1 Deposits related to chemical sedimentation Sedimentary Iron Ores GLY 361 Lecture 14

3 World Iron Ore Production USGS, 2014

4 Crude Steel production

5 Crude Steel production South Africa ranks 21

6 South African Iron-Ore Mines and Deposits

7 South African Iron-Ore Mines and Deposits

8 SA s Main Ore Producers 1. Sishen - Postmasburg area in Northern Cape (Est. Reserves: 4200 Mt) 2. Thabazimbi area in Northwestern Province (Est. Reserves: 100Mt) 3. Bushveld Complex (titaniferous magnetite 55 57% Fe) mainly from Mpumalanga. 4. Beach Sand Deposits (by-product).

9 Sedimentary ores Ore components localized by processes of sedimentation or diagenesis. High tonnage, medium to high grade Sedimentary iron deposits: Banded Iron Formation (BIF) Oolitic ferruginous deposits (e.g. Clinton ores, USA; Minnette ores; Alsace-Lorraine, France) Bog ores Iron carbonate beds (black band ores)

10 Banded Iron Formation

11 Banded Iron Formation Synonyms: Taconite (Lake Superior district), itabirite (Brazil), jaspilite (Australia) 1 billion tons of iron ore is produced and consumed each year. Banded Iron Formations, now serving 99% of the industrial need for iron, were only discovered in the 19th century and initially exploited at the beginning of 20th century. (Bog Iron and ironstone deposits served for a long time as the main source of iron and were the base for the development of the Iron Age and for the beginning of industrialisation of Europe and N-America!)

12 Banded Iron Formation Banded Iron Formations (BIFs) are sedimentary, quartzrich rocks (cherts), with a relatively high Fe content. According to various definitions, BIFs consist mainly of chert, with a minimum of 15% and up to over 50% of FeO, as haematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), siderite (FeCO 3 ) and Fe-silicates and sulfides. Per definition these rocks are banded, e.g. finely laminated and regularly bedded on regional scale. Accessory minerals in BIF are riebeckite (Na), dolomite, stilpnomelane as detritus, and, in metamorphic BIF, acmite, green biotite and other. Chemistry is thus very simple and characteristically lacking K, Al, and other common elements. Most economic iron formations contain 25-35% iron. Regular BIFs are not iron ores!

13 Banded Iron Formation Iron is provided by several characteristic minerals including: Granular magnetite (Fe 3 O 4 ) Hematite (Fe 2 O 3 ) or limonite (Fe 2 O 3 nh 2 O, Fe(OH) 3, FeO OH) Siderite (FeCO 3 ) Chlorite (Fe 6 Si 4 O 10 (OH) 8 ) Greenalite [(Fe 2+,Fe 3+ ) 2-3 Si 2 O 5 (OH) 4 ] Chamosite [(Fe 2+,Mg) 5 Al(AlSi 3 O 10 )(OH) 8 ] Minnesotaite [(Fe 2+,Mg) 3 Si 4 O 10 (OH) 2 ] Grunerite (Fe 7 Si 8 O 22 (OH) 2 ) Stilpnomelane (K(Fe 2+,Mg,Fe 3+ ) 8 (Si,Al) 12 (O,OH) 27 ) The olivine fayalite (Fe 2 SiO 4 ) Ferruginous chert (jasper) (SiO 2 ) Pyrite (FeS 2 ) Pyrrhotite (Fe (1-x) S)

14 Banded Iron Formation BIF mineral facies: Oxide Facies (30-35% Fe) - hematite subfacies - magnetite subfacies Silicate Facies: chamosite, minnesotaite, stilpnomelane Carbonate Facies: siderite (ankerite) Sulphide Facies: pyrite (± pyrrhotite)

15 Banded Iron Formation The stability field of haematite is much larger than that of magnetite that requires low reducing and alkaline (low Eh and high ph). Siderite and pyrite require relatively neutral ph-eh and reducing conditions. BIFs contain mainly haematite, sometimes also high magnetite and some siderite but little to no pyrite. On the other hand, SiO 2 is only stable below ph of 9-10, and goes into solution above it.

16 Banded Iron Formation Three great periods of Precambrian iron formation development: Ma, Algoma BIF Ma, Lake Superior BIF Ma, Rapitan BIF

Banded Iron Formation Three broad classes of BIF: Algoma-type (Archean age; 3.8-2.

17 Banded Iron Formation Three broad classes of BIF: Algoma-type (Archean age; Ga) - related to submarine volcanic processes - within greenstone belts; oceanic volcanic arcs Superior-type (Proterozoic age; Ga) - may include some volcanic input but need not - stable continental platforms and margins Rapitan-type (Proterozoic age; Ga) - in aftermath of global glaciations - passive continental margins and continental setting

18 Banded Iron Formation Models for BIF sedimentation, not including Mn-rich BIF

19 Banded Iron Formation Relative abundance of BIF deposits First giant carbonate platforms Oldest large carbonate platform: Wit Mfolozi 2.9Ga

20 Banded Iron Formation Algoma BIF: closely related to volcanic rocks rarely more than 50 m thick more magnetite than hematite deposits are fairly small but mineable; e.g. Abitibi greenstone belt, Canada; directly connected to volcanic exhalations Different BIF mineral facies: oxide and carbonate facies minerals interlayered with chert, jasper, and finely granular quartz silicate facies is sparse: greenalite-chamositeminnesotaite carbonaceous sulphide facies common

21 Banded Iron Formation Algoma BIF: 40-55% SiO2, 28-37% Fe. Trace elements that might be consistent with volcanic exhalation: Mn, Ba, Co, Ni, Cu, Cr, As, Sr, Au (all in ppm) Adjacent rocks: Graywackes with prominent volcanically derived components Pyroclastic flow units Immediate footwall units may include altered-fractured, or brecciated zones resembling pipes or solution conduits (black smokers)

Banded Iron Formation Lake Superior BIF: gigantic BIF deposits. The most important Fe-ore deposits distributed on Precambrian cratons; Hamersley basin, W.A.

22 Banded Iron Formation Lake Superior BIF: gigantic BIF deposits. The most important Fe-ore deposits distributed on Precambrian cratons; Hamersley basin, W.A.; Transvaal basin, S.A.; Quadrilatero Ferrifero, Brasil; Labrador trough, Canada; Lake Superior district, Canada; Krivoi Rog - Kursk, Ukraine; Singhbhum craton, India, etc. no apparent connection to volcanic setting! several 100 metres thick magnetite still predominant but slightly more hematite than in Algomatype deposits.

23 Banded Iron Formation Rapitan BIF: smaller deposits of large extend but minor thickness (tens of metres): Rapitan Group - McKenzie Mountains, Canada; Scandinavia. Connection to upper Proterozoic glaciation.

24 Banded Iron Formation Greenstone Belt BIF - Submarine Volcanic Spreading Centres Deep Shelf BIF Granular - shallowing reworked GIF Glacial, Snowball Earth BIF

25 Banded Iron Formation Lake Superior BIFs are usually older than 2.2 Ga and show no clastic input or reworking GIFs are usually younger than Lake Superior BIFs and are associated with clastic rocks

26 Banded Iron Formation At least five different theories about the formation of BIF s are discussed in literature: Silica and iron associated with volcanism were poured out on the seafloor from springs of magmatic origin (van Hise and Leith, 1911; Trendall, 1965; Gross, 1980). Iron and silica carried in true solution from nearby landmasses were rhythmically deposited as sediments in water, probably in response to seasonal variations in the composition of the water and involving either direct inorganic precipitation of silica and iron or one of several biochemical processes (Baarghorn and Tyler, 1965; Eugster and I- Ming, 1973).

27 Banded Iron Formation Iron formation beds were originally deposited as more thickly bedded fine-grained ferruginous tuffs and other iron-rich sediments that were diagenetically oxidized and silicified under the influence of solutions that were partly volcanic in origin. The silification caused separation to finer beds of banded cherts and jaspers that alternate with more iron-rich layers (Dunn, 1935, 1941). Iron formations were deposited as end members, or final products, of carbonate sedimentary cycles (Button, 1976).

28 Banded Iron Formation Deposition of iron formation resulted after buildup of iron concentration in the sea. Joliffe (1966) envisioned a primitive Archean acidic sea with a ph of 6 or less, an Eh of about 0, and with seawater in equilibrium with an atmosphere rich in CO 2. Under these conditions, iron released by erosion and by volcanism would remain as ferrous iron in the sea. As time progressed, the CO 2 of the atmosphere was gradually depleted, and an increase in the ph of the sea resulted in removal of H 2 CO 3. A point of saturation was ultimatively reached, and FeCO 3 and Fe 3 O 4 started to precipitate. The gradual buildup of oxygen and the depletion of CO 2 in the atmosphere eventually led to the wholesale precipitation of iron in seawater as magnetite, hematite, and siderite.

29 Banded Iron Formation Depositional environments proposed for BIF: shallow marine, restricted evaporitic biogenic Fe and Si precipitation deep marine basins (shelf) submarine plateaus (deep) ocean spreading centres (or their Archean equivalents) oceanic, connected to global glaciations (post snow ball earth deposits) 3 major types of BIF are distiguished: In submarine volcanic spreading centres, Algoma type In shelf environments below storm wave base, Lake Superior type, including Granular Iron Formations (GIF) - shallowed and reworked BIF) Rapitan BIF, deposited as a consequence of O 2 enrichment after global glaciations. Less common carbonate (siderite) IF Today, it is largely agreed that BIF are deep water deposits. The Fe is of oceanic hydrothermal origin. Si is still a problem and precipitation mechanism for both is not clear, however BIFs bear very little signs for organic influence on precipitation and are regarded as chemical sediments

How was the CO 2 depleted? Where did the oxygen come from?

perhaps the first time in the young Earth's oceans, free O 2 as a waste

The free O 2 oxidized the dissolved Fe 2+ -ions to form insoluble iron

30 How was the CO 2 depleted? Where did the oxygen come from? Photosynthetic bacteria (cyanobacteria) in shallow basins produced, for perhaps the first time in the young Earth's oceans, free O 2 as a waste product. The free O 2 oxidized the dissolved Fe 2+ -ions to form insoluble iron oxides (Fe 3+ is not soluble in water) Precipitating iron oxides formed thin layers on the seafloor substrate (anoxic mud, forming shale and chert).

31 Banded Iron Formation

32 Banded Iron Formation

33 Depositional-environmental facies concept of iron formations: Oxide Facies: Hematite: accumulated in a strongly oxidizing near-shore environment (e.g., Clinton Formation, USA) Magnetite (interlayered with silica, carbonates, iron silicates): weakly oxidizing to moderately reducing conditions (e.g., Lake Superior, USA, Canada).

34 Depositional-environmental facies concept of iron formations: Carbonate Facies: Interbedded siderite, iron-rich ankerite, chert oxygen concentration high enough to destroy most organic material but not high enough to permit ferric compounds to form

35 Depositional-environmental facies concept of iron formations: Sulphide Facies: Black carbonaceous slates with up to 40% pyrite and free carbon content of 5-15% - stagnation and strongly reducing conditions. Deep-water or volcano-slope proximal depressions.

36 Depositional-environmental facies concept of iron formations: Algoma-SVOP: Proximal Superior-MECS: distal Progression from sulfide through carbonate to oxide was considered to reflect precipitation succession from reducing to progressively oxidising conditions (James, 1954). However, all facies occur in BIF together, in the same laminae. Rather, redox potential, alkalinity and depositional environment determine the mineralogy of BIF in a poorly understood way.

Banded Iron Formation Preston Cloud, 1968: Cloud Hypothesis: Archean atmosphere deficient in O 2 and rich in CO 2 (O 2pP < 1%PAL) CO 2 dominated weathering and enrichment of Fe 2+ in the oceans.

37 Banded Iron Formation Preston Cloud, 1968: Cloud Hypothesis: Archean atmosphere deficient in O 2 and rich in CO 2 (O 2pP < 1%PAL) CO 2 dominated weathering and enrichment of Fe 2+ in the oceans. Hematite and magnetite are precipitated upon oxidation (photosynthesis) at 2.5 Ga. Contra - arguments: BIFs are not synchronous BIFs contain little C org Many BIFs are deposited before significant oxygen enrichment

38 Banded Iron Formation Model for BIF deposition on the Kaapvaal craton, South Africa: BIF are precipitated after transgression and drowning of the carbonate platform

39 Banded Iron Formation Model for hematite, magnetite and siderite precipitation via photosynthetic oxidation of Fe 2+. Hematite requires however 1.5 O : 1.0 Fe and magnetite 1.33 O : 1.0 Fe.

40 Banded Iron Formation Iron oxidation and BIF precipitation via: oxygenic and anoxygenic photosynthesis. Haematite is precipitated at sufficient O 2 levels Anoxygenic photosynthesis produces siderite N.J. Beukes, 2003

41 Banded Iron Formation Transition zone with colloidal Fe 3+ brine and oxy-hydroxy-chloride complexes

42 REE patterns like positive Eu anomaly, evidence an oceanic hydrothermal vent source for the Fe in BIF, this influence is slightly weaker in Lake Superior (upwelling model) than in Algoma BIF and disappears in the Rapitan BIF, where the Fecontent appears to be continental.

43 It has been demonstrated that continental weathering can easily supply the required mass of Fe needed to precipitate world wide BIF deposits. Precipitation of silica is another problem in BIF precipitation, as no biological SiO 2 precipitation is known in the Precambrian. Silica is however, increasingly soluble with increasing alkalinity. Amorphous silica [Si(OH) 4 ] is nevertheless highly soluble (~120g/l) and Precambrian oceans might have been supersaturated in silica in the absence of SiO 2 secreting organisms, leading to colloidal or gel precipitation of SiO 2. Silica precipitation could also be driven by seasonal alkalinity changes in the surface sea water.

44 Banding Banding is assumed to result from cyclic variations in available oxygen. At tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition. For reasons largely unknown, this was a periodic process resulting in the alternating bands of iron oxide and shale. The periodic process might have been due to seasonal fluctuations or storm surges.

45 Banding

46 Banding Dales Gorge BIF

47 Banding Bedding and banding: Trendall & Blockley, 1970: Macrobands: in m - range; BIF-macrobands are intercalated with S-macrobands (stilpnomelane). Each BIF-macroband consists of an intercalation of Fe-rich and -poor mesobands. Mesobands: cm - range; Chert with abundant SiO 2 -Chert matrix (and subordinate FeO) and SiO 2 (with more FeO). Some mesobands can be composed of microbands. Microbands: mm - range; alternating lamination of hematite, magnetite, carbonate, stilpnomelane or any combination of these minerals with chert. Microbands are about 2.0 to 0.2 mm thick. AV =Acidic Volcanics; C-M = Chert matrix Microbands were interpreted by AF Trendall (AFTbands) as varvites or nocti-diurnal laminae.

48 Banding Fecarbonates Chert matrix Chert Mesoband Fe-Oxide Fe-Hydrooxide (Weathering to limonite) Microbands 1 cm

BIF Alteration Low grade metamorphism of BIF creates veins of fibrous or asbestos riebeckite. Often these asbestos type crystals are replaced by quartz.

49 BIF Alteration Low grade metamorphism of BIF creates veins of fibrous or asbestos riebeckite. Often these asbestos type crystals are replaced by quartz. The formerly fibrous nature of the crystals causes the play of light that is known as Tiger's Eye. Due to the extreme age of these formations, almost all BIF formations have undergone some faulting, fracturing, folding, compaction, veining, intrusions and metamorphism. Although all BIF formations are probably metamorphosed to some degree their general character is still sedimentary.

50 BIF Alteration Original BIF precipitates and metamorphic equivalents: Compound Inferred initial precipitate SiO 2 Amorphous Chert Fe 2 O 3 Fe 3 O 4 Amorphous Fe 2 O 3 nh 2 O Fe 3 O 4 nh 2 O hydromagnetite Now observed Hematite Magnetite FeCO 3 Siderite Siderite Fe 3 Si 2 O 5 (OH) 4 Fe sulfide Amorphous ferrous silicate FeS Fe 3 Si 2 O 5 (OH) 4 greenalite Fe 7 Si 8 O 22 (OH) 2 grunerite Fe3Si4O10 Na-Fe silicate Na-Fe silicate gels

51 Secondary enrichment proto-ore may hardly be regarded as iron ores as the silica/iron ratio of these rocks is too high. iron content of the BIF s varies between 25-35%. A secondary natural process is therefore an essential requirement in the formation of the ores. supergene/ hypogene enrichment upgrades the proto-ore to above 60% Fe.

52 Origin of Fe-ore in Banded Iron Formations, supergene vs. hypogene mineralisation Sishen, Kathu supergene deposits Bello Horizonte, hypogene deposits

53 Thabazimbi Supergene Hypogene

54 Supergene vs. hypogene mineralisation Enrichment of Fe in BIF must be at least 100% to make it a mineable deposit. Sishen/Kathu in SA and Mount Whaleback in WA are the world largest single iron deposits with c % FeO. Mount Whaleback alone produces ~ 170 Mio.t/annually, exported to Japan, China and S. Korea, with an resource of 1.8 billion t. The ore hosted in BIF and shales of the Dales Gorge Member of the Brockman BIF, is interpreted as supergene enrichment after folding during the Ophthalmian orogeny (~ Ma) and subsequent rising and exposure. This correlates to the syndepositional folding of the Kuruman BIF in SA, and exposure during the Paleoproterozoic. The BIFs were exposed and subject to supergene alteration with martitization (oxidation of magnetite to hematite) prior to 2200Ma (age of the Wyloo Group conglomerates, and Ongeluk/Hekpoort)

55 Supergene vs. hypogene mineralisation After supergene alteration the goethite-martite was buried and metamorphosed to microcrystalline hematite-martite ore (e.g. Pretoria Group sedimentation) resulting in lumpy, low P (0.05%), and high Fe (>65%) ore (Morris, 1985). However, fluid inclusions and stable isotope data evidence hematitemartite ore formation above 400 C which is incompatible with supergene, near-surface conditions, supported by mineralogy evidencing high P/T conditions. Mineralisation by hypogene fluids along low angle thrusts has been therefore suggested (Powell et al., 1999; Barley et al., 1999). This is supported by occurrence of Fe ores down to a depth of 500m below the surface, far too deep for supergene enrichment. Thus, orogenic, hot, oxidising, basinal fluids were proposed as the mineralising medium.

Post-Precambrian sedimentary iron deposits Oolitic

scattered throughout a clay or limestone matrix.

Oolites of hematite, limonite, siderite, chamosite,

56 Post-Precambrian sedimentary iron deposits Oolitic iron ores: Can make up most of the rock or may be scattered throughout a clay or limestone matrix. Most valuable Post-Precambrian deposit. Oolites of hematite, limonite, siderite, chamosite, with or without calcite or chalcedonic silica. e.g., Clinton Formation (Silurian age).

57 Ironstone deposits (Alsace-Lorraine or Minette - type ores): Ironstones, oolitic and peloidal Fe ores are widespread through certain periods of the Phanerozoic era and were an important source for iron ore in Europe (England, France, Germany) and are still mined in eastern USA (Kentucky, Alabama). The Silurian-Ordovician and Jurassic-Cretaceous (periods of very low continental freeboard) deposits formed in shallow marine to deltaic environments, above lateritic ferricretes, through abrasion, mechanical reworking to oolites and pellets, and through bacterial precipitation at oolite surface. They are often associated with glauconite (K,Na,Ca) (Al, Fe 2+, Fe 3+, Mg) 2 [(OH) 2 Al 0.35 Si 3,65 O 10 ] and chamosite (Fe-rich chlorite). Continental source of Fe, drowned at transgressions (continental flooding, sealevel high stand) and laterally linked to deeper water black shales. Or marine origin, connected to Ordovician-Silurian or Jurassic sea level high stands and upwelling.

58 Ironstone deposits (Alsace-Lorraine or Minette - type ores): Reworking of lateritic iron deposits and upwelling of Fe-rich, deep oceanic waters at times of pronounced sea-level highstand and mid ocean ridge activity during continental dispersion. Deposition of continental reworked hematitic and goethitic ores and Fe-oolite formation through biological processes in shallow water (bacterial oxidation of Fe)

59 Post-Precambrian sedimentary iron deposits Black band ores (siderite): widely distributed throughout the world. generally of low grade (successfully mined in Germany, England, USA).

60 Post-Precambrian sedimentary iron deposits Bog ore

61 Post-Precambrian sedimentary iron deposits Bog ores and spring deposits: Limonitic iron ores. Occur in small low-grade deposits with Mn, P, water, clay, etc. Not mined. Good example for biochemical precipitation of iron minerals: iron content in bog waters is higher than that of other surface waters because iron is stabilized by humic complexes and low ph. bacterial action causes precipitation of ferric oxides and hydroxides from breakdown of humic iron complexes and ferrous bicarbonate. iron is delivered by streams and springs.

62 Bog iron ore deposits: Formed in geologically young and recent swamps and lakes of the interglacials of northern hemisphere and northern Canada, Scandinavia and arctic Russia. Deposits are typically small and thin, comprising Feoxyhydroxides (goethite - afeooh, limonite g,dfe(oh) 3, and related minerals) and associated laterally with reduced carbonaceous black shales. Fe concentration occurs when Fe 2+ (ferrous iron) is oxidised to Fe 3+ (ferric iron) and precipitated as limonite or goethite from a relatively reducing meteoric water, at contact to relatively oxic ground water. This usually happens at the ground water table. Bacterial oxidation of Fe, may play an important role in these near surface environments.

63 Reducing and acidic meteoric water Meteoric waters in swamp environment are reducing because of oxidation of C org. ph is usually slightly acidic due to humic acids. Neutral or slightly alkaline, oxygenated ground waters cause transformation from ferrous to ferric iron and precipitation.

64 Exploration guide for BIFs GEOCHEMICAL SIGNATURE: Elevated values for Fe and Mn; at times elevated values for Ni, Au, Ag, Cu, Zn Pb, Sn, W, REE and other minor elements. GEOPHYSICAL SIGNATURE: Electromagnetic, magnetic, and electrical conductance and resistivity survey methods are used effectively in tracing and defining the distribution of Algoma- type beds, either in exploring for iron and manganese ore, or for using these beds as metallogenetic markers. OTHER EXPLORATION GUIDES: Discrete, well defined magnetite and hematite lithofacies of ironformation are preferred with a minimum of other lithofacies and clastic sediment interbedded in the crude ore. Iron- formations are usually large regional geological features that are relatively easy to define. Detailed stratigraphic information is an essential part of the database required for defining grade, physical and chemical quality, and beneficiation and concentration characteristics of the ore. Basin analysis and sedimentation modeling enable definition of factors that controlled the development, location and distribution of different iron-formation lithofacies.

1. Lutsinge B 2. Muravha E 3. Moswathupa H

1. Lutsinge B 2. Muravha E 3. Moswathupa H Introduction Occurrence Origion and Formation Iron ore mining processing Economic significance & Uses Conclusion Refference list Iron deposits are natural concentrations